The invention relates to a high-silicon-content corrosion-resistant austenitic steel and its use for the handling of strongly oxidizing media, such as hot highly concentrated sulphuric acid and hot highly concentrated nitric acid.
More particularly for the handling of highly concentrated hot nitric acid, the steel X2CrNiSi1815 was developed, which contains 3.7 to 4.3% silicon in addition to 17 to 18% chromium and 14.5 to 15.5% nickel (all details in % by weight). High resistance to corrosion in superazeotropic, more particularly highly concentrated nitric acid can be achieved only by a minimum silicon content of 3.7% (E. M. Horn, A. Kugler, Z. Werkstofftechnik, Vol. 8, 1977, pages 362 to 370, 410 to 417). In that case the chromium content is approximately I8%, so that passivation can take place in other aqueous solutions also. The relatively high nickel content of approximately 15% is necessary to achieve an austenitic base structure. The effect of higher silicon contents than approximately 4% was also investigated in the past (E. M. Horn, R. Kilian, K. Schoeller, Z. Werkstofftechnik, Vol. 13, 1982, pages 274 to 285). German OS 28 22 224 discloses a steel containing 2.5 to 5% silicon, 15 to 20% chromium, 10 to 22% nickel, max. 2% manganese, max. 0.1% carbon and additions of a further alloying component consisting of tantalum, zirconium or a mixture of niobium and tantalum and/or zirconium for the production of corrosion-resistant spring plates. British Patent 2 036 077 discloses inter alia an austenitic steel of improved resistance to oxidation at elevated temperatures which consists of 1 to 5% silicon, 15 to 30% chromium, 7 to 35% nickel, not more than 3% manganese, max. 0.10% carbon, residue iron and impurities, the sulphur content also being limited to max. 0.003%. A steel is also commercially available which has a silicon content raised to 5 to 5.6%, the nickel content being increased to approximately 17.5%, to enable an austenitic structure still to be established. British Patent 2 122 594 claims the use of such a steel for parts of installations required for the production of sulphuric acid. Nevertheless, as a rule in the prior art no higher silicon content than approximately 4.5% is selected, since with chromium contents of approximately 18% the precipitation of carbides and intermetallic phases as a whole is accelerated by increasing silicon contents.
The steel containing approximately 4% silicon is included under Case 1953 in the ASME Boiler und Pressure Vessel Code, Sect. VIII, Div. 1. The strong tendency to precipitation demands inter alia special steps during welding (R. R. Kirchheiner, F. Hofmann, Th. Hoffmann, G. Rudolph, Materials Performance, Vol. 26, No. 1, 1987. pages 49-56). Furthermore an austenitic steel containing 3.5 to 4.5% silicon, 16 to 18% chromium, 8 to 9% nickel, 7 to 9% manganese, max. 0.10% carbon and 0.08 to 0.18% nitrogen is offered on the market as a particularly wear-resistant material under the name Nitronic 60.
In addition to the aforementioned austenitic silicon-containing Steels, European patent 0 135 320 discloses a silicon-containing austenitic-ferritic steel which is supposed to be particularly suitable for the handling of such solutions of nitric acid as are used in the processing of nuclear reactor fuel elements. Its composition is stated as 2 to 6% silicon, 20 to 35% chromium, 3 to 27% nickel, 0.1 to 2% manganese, max. 0.03% nitrogen, max. 0.04% carbon, at least one of the elements niobium, titanium or tantalum in a quantity 8 times the carbon content or more, but at most 1%, residue mainly iron. With a view to the same field of application, European Patent 0 135 321 discloses a silicon-containing austenitic steel having improved resistance to corrosion caused by nitric acid, its composition being stated as follows: 2 to 6% silicon, 20 to 35% chromium, 17 to 50% nickel, 0.01 to 8% manganese, max. 0.03% nitrogen, max. 0.03% carbon, at least one of the elements niobium, titanium and tantalum in a quantity 8 times the carbon content or more, but 1% at most, residue mainly iron.
However, an overall consideration of the aforementioned silicon-containing corrosion-resistant steels shows that even with Si contents up to 6%, resistance is inadequate in highly concentrated hot sulphuric acid at temperatures above 100.degree. C., taking in account a maximum corrosion rate of 0.3 mm per annum, which is tolerable for practical applications.
According to British Patent 1 534 926 a corrosion rate lower than 0.3 mm per annum, tested in 95.6% sulphuric acid at 110.degree. C., can be achieved with the following alloy composition: 4.1 to 12% silicon, 6 to 22% chromium, 10 to 40% nickel. 0.6 to 4% copper, max. 4% manganese, max. 1.5% molybdenum plus 1/2 tungsten, max. 0.2% nitrogen, max. 0.06% carbon, total max. 2% for the elements niobium, tantalum, zirconium and vanadium, residue mainly iron. According to this Patent Specification the optimum silicon content is normally supposed to be 7.5 to 10%, the chromium content being preferably 9 to 14%, the nickel content preferably 14 to 20% and the copper content 2 to 3%.
However, at test temperatures of 150.degree. C. and above, the corrosion rates appreciably exceed the limit value of 0.3 mm per annum relevant in practice, as tests carried out at a neutral Institute showed in the testing of commercially available steels having the composition stated in the analysis given in British Patent 1 534 926. In those tests the most favourable corrosion rate in 96% sulphuric acid at a test temperature of 150.degree. C. was 0.5 mm per annum.
Moreover, due to its high silicon content in combination with the copper content this steel is difficult to work, so that rolled products of relatively large dimensions, such as plates and pipes, can be produced only to a limited extent. To improve hot workability, a total of up to 0.5% magnesium, aluminium and calcium and also up to 0.2% rare earth metals must be added to that steel.
Starting from this prior art it is an object of the invention to provide a satisfactorily workable silicon-containing austenitic steel which can be processed into rolled products of relatively large dimensions, such as plates and pipes, and which is adequately corrosion-resistant for practical use in the handling of highly concentrated hot sulphuric acid, highly concentrated hot nitric acid and other strongly oxidizing media (rate of corrosion below 0.3 mm per annum).
This problem is solved by an austenitic steel having alloying contents of max. 0.2% carbon, 10 to 25% nickel, 8 to 13% chromium, 6.5 to 8% silicon, 0 to 10% manganese and/or cobalt, max. 0.010% sulphur, max. 0.025% phosphorus, residue iron and usual admixtures and impurities due to manufacture (all details in % by weight).